Spanish Model 1916, and your serial number falls into the ones converted to 7.62 NATO (not .308Win), and more suited for the 7.62 CETME round. Do the bubba pencil test, standard yellow pencil will stick in the muzzle of a 7mm, falls into a 7.62.
Any pre 98 small ring Mauser follower and spring will work in the mag, I'll check the spare parts box and see if I have one. Before you start messing with it, two things need to be done. Have the head space checked, and have it checked for lug setback at the same time. Most of these were advertised as .308Win, and most had that higher pressure round ran through them. Enough of it, and setback will be present, meaning it's unsafe to shoot without a lot of expensive gunsmith work.
If that checks out, surplus CETME ammo, or maybe some of the new low pressure .308, but I would learn to reload and use .300 Savage loading data.